Add Release Guidelines to document release process

[frtibble]

Part of #651

Description

Introducing versioning means the addition of several new (to the repo) concepts such as conventional commit messages and semantic versioning. Documentation needs to be included which explains what these are and how they are used, e.g. what is considered a breaking change.

Additionally, the changes which introduce versioning need to be explained so that anyone coming to the repo could understand how it works, e.g. how GitHub actions creates a release PR, how to do a manual release, etc.

Tasks

• Add docs